Characterization of meta -Cresol Purple for spectrophotometric pH measurements in saline and hypersaline media at sub-zero temperatures

urate pH measurements in polar waters and sea ice brines require pH indicator dyes characterized at near-zero and below-zero temperatures and high salinities. We present experimentally determined physical and chemical characteristics of purified meta-Cresol Purple ( m CP) pH indicator dye suitable for pH measurements in seawater and conservative seawater-derived brines at salinities (S) between 35 and 100 and temperatures (T) between their freezing point and 298.15 K (25 °C). Within this temperature and salinity range, using purified m CP and a novel thermostated spectrophotometric device, the pH on the total scale (pH T ) can be calculated from direct measurements of the absorbance ratio R of the dye in natural samples as pH_T=-log(k_2^Te_2)+ log(R-e_1/1-Re_3/e_2) Based on the m CP characterization in these extended conditions, the temperature and salinity dependence of the molar absorptivity ratios and − log(k_2^Te_2) of purified m CP is described by the following functions: e 1 = −0.004363 + 3.598 × 10 −5 T , e 3 / e 2 = −0.016224 + 2.42851 × 10 −4 T + 5.05663 × 10 −5 ( S − 35), and − log(k_2^Te_2) = −319.8369 + 0.688159 S −0.00018374 S 2 + (10508.724 − 32.9599 S + 0.059082 S 2 ) T −1 + (55.54253 − 0.101639 S ) ln T −0.08112151 T . This work takes the characterisation of m CP beyond the currently available ranges of 278.15 K ≤ T ≤ 308.15 K and 20 ≤ S ≤ 40 in natural seawater, thereby allowing high quality pH T measurements in polar systems.